The present invention relates to a wire section of a fiber web forming machine, particularly a paper making machine for forming a multi-ply fiber web, particularly a paper web. The wire section includes a wire section belt of a paper machine on which a first fiber ply is formed. It includes a twin-wire part of the wire section designed as a gap former and having a first and a second wire in which part a second fiber ply is formed. The two wires wrap around a forming roll at the beginning of the twin wire part. It further includes a combining section, in which the first and the second fiber plies are combined, for forming the multi-ply fiber web.
The invention further relates to a process for forming a multi-ply fiber web, including the steps of forming a first fiber ply, forming a second fiber ply, and then combining the first fiber ply, which is running in on a belt, and the second fiber ply, which is running in on a first wire, in a combining section.
Such a wire section and a process of this type for forming a multi-ply fiber web are disclosed in DE 44 02 274 A1, equivalent to U.S. Pat. No. 5,584,967. This known wire section comprises a conventional Fourdrinier unit for forming a first fiber ply on a belt in the form of a horizontal wire. A second fiber ply is formed by a twin-wire part arranged above the first belt. The first and second plies are couched together, forming a multi-ply fiber web, particularly a paper or board web. According to FIG. 5 of DE '274, the twin-wire part is designed as a gap former.
The twin-wire part for forming the second fiber ply has a headbox or flowbox, has an evacuated forming roll downstream of the headbox, has a so-called D part which typically dewaters the web through a wire by suction and also applies pressure pulses on the wire and has a second forming roll. The two wires of the twin-wire part are led approximately horizontally and counter to the running direction of the belt, between the first forming roll and the second forming roll.
In the outlet region of the second forming roll, the top wire is lifted off the second fiber ply, and the second fiber ply is led to the couch roll on the bottom wire, at an angle of about 80.degree. to the first wire.
Another wire section for forming a multi-ply fiber web is disclosed in WO 92/01111. In this wire section too, a first fiber ply is brought up on a belt which is a wire belt that runs approximately horizontally. A twin-wire part for forming a second fiber ply is arranged above the belt. The twin-wire part for forming the second fiber ply has a headbox and a forming board arranged downstream of the headbox. The board has a multiplicity of forming foils, which form a convexly slightly curved running surface for the first and the second wires and which engage the wire to produce pressure pulses. A wiper is provided on the top side at the outlet of the forming board. The top wire is lifted off the second fiber web upstream of the entry region of a couch roll. The bottom wire wraps around the couch roll by about 120.degree.. A guide roll is provided on the underside of the belt so that the belt and the first wire wrap around the couch roll by about 45.degree.. In the wire section in WO 92/01111, dewatering of the second fiber ply takes place solely on account of the tensile stress of the wires acting on the forming board, by centrifugal forces and by the force of gravity.
It is not possible to achieve high running speeds using these known wire sections. In addition, the twin-wire part arranged above the Fourdrinier unit needs considerable space. It is particularly unfavorable that the twin-wire part is located above that part of the Fourdrinier unit in which the finally formed (but still moist) multi-ply fiber web runs, on the belt, in the direction toward the following treatment stations (e.g. wire suction roll, press section, etc.). The quality of the web is thereby impaired.